Contact methods (e.g. Goldmann tonometry, Mackay-Marg tonometry) for measuring IOP mostly involve use of an anesthetic to carry out the measurement and are thus impractical for screening large human populations. Also non-contact air impulse tonometers have been on the market for decades. Air impulse tonometers can result in patient discomfort experienced due to an air impulse.
US patent application document US 2010/0249569 A1 presents a non-contact ultrasonic tonometer for IOP measurements, which employs piezo-electric transducers to excite wave signals into the eye. The positions of the transducers have to be exactly measured, which makes the IOP measurement procedure complex and slow. Also temperature variations cause error and uncertainty in the IOP measurement information together with possible errors in position measurements. The eye shape can also introduce bias, i.e. error, into the measurement.
Patent document U.S. Pat. No. 6,030,343 A presents a method that is based on an airborne ultrasonic beam that is reflected from the cornea—the same beam measures and actuates the eye. The actuation is done by a narrow band ultrasonic tone burst, which deforms the cornea, and the system measures the phase shift from the deformed eye.
Patent documents US2004/193033 and U.S. Pat. No. 5,251,627 describe a non-contact measurement method by way of linear excitation (e.g. loudspeakers or ultrasonic transducers).
Known solutions have difficulty providing a convenient and low-cost device for measuring IOP precisely and comfortably for a patient by non-contact measurements.